The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure actuated device, such as a steering cylinder for steering a vehicle. More particularly, the present invention relates to such a steering system having both a normal input, such as the vehicle steering wheel, and an auxiliary input, such as a joystick.
Although the present invention may be used in connection with fluid controllers of many types, and having various applications, it is especially advantageous when used in conjunction with a full-fluid-linked steering system, and will be described in connection therewith.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter, a valve means, and an arrangement for imparting follow-up movement to the valve means, in response to the flow of fluid through the fluid meter. The flow through the controller valve means is directly proportional to the area of variable flow control orifices in the main fluid path, the area of the flow control orifices in turn typically being proportional to the rate at which the steering wheel is rotated.
A typical application for a full-fluid-linked steering system of the type to which the present invention relates would be a vehicle such as is used on a construction site. Such a vehicle is used primarily in one of two operating modes: first, the vehicle is driven in a "roading" mode, i.e., it is being driven on the road, at normal roading speeds, in order to reach the work site. Secondly, the vehicle is operated in a working mode, on the work site, and performing operations such as moving a pile of dirt, etc.
The roading and working modes of operation described above present very different steering requirements, as is now well known to those skilled in the art. When roading the vehicle, a relatively low gain rate is required, whereas, when operating in the working mode, a relatively high gain rate is required. As used herein, the term "gain rate" refers to the rate of change of steered wheel position for a given amount of steering input. Those skilled in the art are well aware that a relatively high gain rate steering system would be undesirable when roading the vehicle, because it could result in oversteering, possibly causing loss of control of the vehicle. Conversely, a relatively low gain rate steering system is undesirable when the vehicle is in the working mode, because excessive rotation of a steering wheel will quickly lead to operator fatigue, and less productivity on the work site.
Those skilled in the art have for some time recognized that it is desirable to now provide the steering operator with a steering wheel for use when the vehicle is in the roading mode, while providing the operator with a joystick for use when the vehicle is in the working mode. The steering wheel gives the operator somewhat the same feel as driving a car, which is desirable for the roading mode, while the joystick may be used to provide relatively large steering changes, with relatively little operator input, which is desirable for use in the working mode.
One known, commercially available steering system includes a conventional steering control unit, which receives an input from the steering wheel, and a separate, electrically actuated proportional flow control valve, which receives an input by means of a joystick. The output of both the steering control valve and the proportional valve goes to the steering cylinder in a parallel arrangement. There is no interface between the two subsystems (i.e., the one controlled by the wheel, and the other controlled by the joystick), or any ability for the flow output of one subsystem to be augmented by flow output from the other subsystem.
One of the disadvantages of known steering wheel/joystick systems is the complication, and expense of coordinating the portion of the system operated by the steering wheel with the portion of the system operated by the joystick. As one example, if the operator is steering by means of the joystick, and then begins to rotate the steering wheel, there must be logic or some sort of interface provided to determine whether the steering wheel input or the joystick input would take precedence. As a second example if the vehicle is operating above a predetermined vehicle speed, or operating above a predetermined transmission gear, it may be desirable, or even legally required, to prevent steering in response to the joystick input, and permit the operator to steer only by means of the steering wheel.
U.S. Pat. Nos. 5,016,672 and 5,115,640, both of which are assigned to the assignee of the present invention and incorporated herein by reference, disclose fluid controllers in which the valve means, comprising a spool and sleeve, can be actuated by rotation of a steering wheel in the conventional manner. In addition, the sleeve valve may be moved axially, relative to the spool valve, to define an auxiliary fluid path. Control of the axial position of the sleeve valve is by means of a pilot pressure chamber adjacent one axial end of the sleeve valve, the pressure in the pilot pressure chamber being controlled by a solenoid valve. Although the steering system and controller disclosed in the above-incorporated patents provide greatly enhanced performance, it has been found that the pilot control of the axial position of the sleeve valve may be too slow for the type of interface required between steering wheel input and joystick input. It has also been found that variations in fluid temperature and viscosity may make it difficult for such a pilot control system to provide accurate, repeatable performance. In addition, the controller of the above-incorporated patents, utilizing pilot control, does not readily provide axial position feedback of the sleeve valve.